Interpretive Summary: The activity budgets of large herbivores may dictate how these animals impact their environment and environmental factors such as topography, climate or moon phase may in turn influence animal activity. We tested the commonly assumed by as yet untested hypothesis that large grazing animals will forage more during moon-lit nights than dark, moon-less nights. Using GPS tracking collars to monitor the 24-hr activity budgets of adult female, Rocky Mountain elk (Cervus elaphus) during full-moon and new-moon periods in the summer and fall of 2008, we found no differences in distance traveled at night or in mean night travel velocities between moon-phase period types. Though limited, these findings contradict commonly-held beliefs that elk are more active during moon-lit nights suggesting some of our elk management and research reference documents may need to be revisited.

Technical Abstract:
Rocky Mountain elk (Cervus elaphus) exist in a complex biological and social environment that is marked by necessary diurnal activities such as foraging, ruminating, and resting. It has long been understood that elk demonstrate circadian rhythms. One of the most predictable variables that could affect an elk’s environment is night-time illumination by the moon. Moonlight is moderated by clouds and atmospheric conditions; however, it regularly alters night-time visibility. We hypothesize that elk exhibit different innate diurnal movements and spatio-behavioral activities in new and full moon periods. We studied 24-hour foraging and travel activity budgets during full moon and new moon periods in the summer and fall of 2008. Our objective was to quantify: 1) Daily distance traveled by hour throughout the day and night, 2) Day travel, 3) Night travel, 4) Time spent moving at specific velocities. The study pastures are located 35 km west of La Grande, Oregon (45.2422°N, 118.5124°W) and consisted of two pastures: Cuhna West (16.6 ha) and Cuhna East (17.1 ha). During a new moon elk traveled an average of 5073 m/day with 4210 m occurring during daylight and 863 m occurring at night (end of civil twilight to the beginning of civil twilight the next morning). During a full moon, elk traveled an average of 4807 m/day with 3938 m occurring during daylight and 869 m occurring at night. Mean night travel velocities averaged by hour were similar between full and new moon periods (new moon = 118.2 m/hr, full moon = 116.3 m/hr, P=0.8573).